1. Field of the Invention
This invention concerns polymeric and non-polymeric binders for fibers and the use of such binders in binding particles to fibers. The fibers are then densified by external application of pressure. In particular embodiments, the invention concerns binding superabsorbent particles to cellulosic fibers which may then be used, for example, to make absorbent fibers that are densified and incorporated into cellulosic products.
2. General Discussion of the Background
Superabsorbent polymers have been developed in recent years that are capable of absorbing many times their own weight of liquid. These polymers, which are also known as water insoluble hydrogels, have been used to increase the absorbency of sanitary products such as diapers and sanitary napkins. Superabsorbent polymers are often provided in the form of particulate powders, granules, or fibers that are distributed throughout absorbent cellulosic products to increase the absorbency of the product. Superabsorbent particles are described, for example, in U.S. Pat. No. 4,160,059; No. 4,676,784; No. 4,673,402; No. 5,002,814; and No. 5,057,166. Products such as diapers that incorporate absorbent hydrogels are shown in U.S. Pat. No. 3,669,103 and No. 3,670,731.
One problem with the use of superabsorbents is that the superabsorbent material can be physically dislodged from the cellulosic fibers of an absorbent product. Separation of the superabsorbent from its substrate reduces the absorbency of the product and diminishes the effectiveness of the superabsorbent material. This problem was addressed in European Patent Application 442 185 A1, which discloses use of a polyaluminum chloride binder to bind an absorbent polymer to a fibrous substrate. The polyaluminum binder, however, suffers from the drawback of being an inorganic product that is not readily biodegradable. Moreover, that European patent does not offer any guidance for selecting binders other than polyaluminum chloride that would be useful in binding absorbent particles.
A method of immobilizing superabsorbents is disclosed in U.S. Pat. No. 4,410,571 in which a water swellable absorbent polymer is converted to a non-particulate immobilized confluent layer. Polymer particles are converted to a coated film by plasticizing them in a polyhydroxy organic compound such as glycerol, ethylene glycol, or propylene glycol. The superabsorbent assumes a non-particulate immobilized form that can be foamed onto a substrate. The individual particulate identity of the superabsorbent polymer is lost in this process. The confluent nature of the superabsorbent material can also result in gel blocking, in which absorption is diminished as the water swollen polymers block liquid passage through the film layer.
U.S. Pat. No. 4,412,036 and No. 4,467,012 disclose absorbent laminates in which a hydrolyzed starch polyacrylonitrile graft copolymer and glycerol mixture is laminated between two tissue layers. The tissue layers are laminated to each other by applying external heat and pressure. The reaction conditions form covalent bonds between the tissue layers that firmly adhere the tissue layers to one another.
Numerous other patents have described methods of applying binders to fibrous webs. Examples include U.S. Pat. No. 2,757,150; No. 4,584,357; and No. 4,600,462. Such binders are not described as being useful in binding particulates, such as superabsorbent particles, to fibers. Yet other patents disclose crosslinking agents such as polycarboxylic acids that form covalent intrafiber bonds with individualized cellulose fibers, as in European Patent Application 440 472 A1; European Patent Application 427 317 A2; European Patent Application 427 316 A2; and European Patent Application 429 112 A2. The covalent intrafiber bonds are formed at elevated temperatures and increase the bulk of cellulose fibers treated with the crosslinker by forming intrafiber ester crosslinks. Crosslinking must occur under acidic conditions to prevent reversion of the ester bonds. The covalent bonds within the fibers produce a pulp sheet that is more difficult to compress to conventional pulp sheet densities than in an untreated sheet. Covalent crosslink bonds may also form between the fibers and particles, and occupy functional groups that would otherwise be available for absorption, hence absorption efficiency is decreased.
A particular disadvantage of forming covalent ester intrafiber crosslinks is that the resulting fiber product resists densification. Energy requirements for making densified absorbent products are increased because very high compression pressures must be used to densify the absorbent product. Such high pressure can also damage the fibers. Antimicrobials, zeolites and fire retardants are but a few examples of particles that are added to fibers. It would be advantageous to provide a method of attaching particles that could be accommodated to the many different particle needs of end users. Moreover, it would be advantageous to reduce particulate waste in the attachment process, and simplify shipment of fiber products that require particulate addition.
Accordingly, it is an object of this invention to provide an improved method of densifying fibers, including high bulk fibers that have intrafiber crosslinks.
It is also an object of this invention to provide an improved method of densifying fibers, including superabsorbent particles, to fibers.
It is another object to provide an improved method of binding particulates such that they can be distributed throughout a crosslinked fibrous product in a desired distribution and without necessarily being confined to the surface of a product.
Another object of the invention is to provide improved fiber and absorbent products in which particulates are firmly bound to cellulose fibers such that the particles are less likely dislodged by mechanical forces.
Yet another object of the invention is to provide an improved particle binder that is environmentally compatible and more easily biodegradable.
Finally, it is an object of the invention to bind a broad variety of particles to many different kinds of fibers using an improved, simple and versatile binding process that limits particle waste.